1. Field of the Invention
The present invention concerns the direct digital generation of Linear Frequency Modulation (LFM) signals such as are applicable to radars.
2. Background of the Invention
Frequency-Modulated (FM) radar is a form of radar in which the radiated wave is modulated in frequency and the returning echo beats with the wave being radiated, thus enabling the range to be measured. The use of wideband Linear Frequency Modulation (LFM) coded waveforms in radar for purposes of pulse compression is a well known technique. In peak-power-limited radar transmitters, the use of LFM permits an increased amount of energy to be transmitted in a radar pulse by increasing the pulse duration while maintaining the range resolution, or signal bandwidth, of a shorter pulse.
Wideband LFM waveforms are applicable to high resolution Synthetic Aperture Radars, and to target identification and target counting modes in multi-mode airborne radars. These radars generally require broadband microwave signals in order to measure range profiles of targets of interest and opportunity. For example, and regardless of the waveform modulation format, a range resolution R requires a transmitted bandwidth B of at least B=aC/2R, where "C" is the propagation velocity of light and "a" is a constant (usually 1.0 to 1.2) related to signal processing weighting. Moreover, radar systems using large signal bandwidths may often use "stretch" waveforms as described by W. Caputi in "Stretch: A Time Transformation Technique", appearing in IEEE Transactions on Aerospace and Electronic Systems, Volume AES-7, March 1971. Stretch waveforms can reduce intermediate frequency (IF) and signal processing bandwidths by transmitting waveforms of duration longer than the range swath collection time. Stretch radar systems typically require waveforms of 300 MHz bandwidth and 500 microsecond time duration on a carrier frequency which is typically X band in the order of 10 GHz. For such a radar system the bandwidth-time product BT is thus 150,000, and the waveform slope K is 6.times.10 Hz/second. LFM waveforms satisfy these requirements.
Another requirement sometimes placed on LFM waveforms in certain radar applications is that the transmitted pulse, or LFM waveform, should be variable in bandwidth and duration to match the functional requirements of a selected operating mode. A signal processing matched filter required to obtain this variability is frequently realized by a digital signal processor. A digital signal processor is capable of processing signals with a wide range of bandwidth-time, BT, products. Moreover, the exciter of certain radar systems must also be flexible, which again leads to the flexibility of digital waveform generation techniques.
Still another requirement of LFM waveforms -- in addition to the required large BT product and the required flexibility in generating both B and T -- is that residual generation errors should be very low. This is so that the side lobes of large target returns do not mask weaker target returns. An amplitude weighting of the received radar return signals, reducing side lobes to less than -35 dB, is frequently used in radar systems. Hence, the errors in the LFM waveform generation must be less than -35 dB.
A first prior art approach to generation of a LFM waveform sweeps the frequency of an X band carrier wave radar signal in a Voltage Controlled Oscillator (VCO). That approach is called the linearized swept-VCO approach. The approach requires complex error correction loops in order to produce high accuracy, low distortion, LFM waveforms. The error correction loops rely on a delayed sample of the VCO output signal to determine frequency linearity. Absolute or fixed external references are not used. Hence, a potential for residual errors exists in this approach.
A second prior art approach to the generation of LFM waveforms is described in U.S. Pat. No. 4,160,958. This approach uses a stable frequency source of a carrier signal which is adjusted in phase by a binary phaser. A tuned Voltage Controlled Oscillator (VCO) is locked to the adjusted phase source via a sampled phase-locked loop. This sampling is at microwave frequencies. The instantaneous phase of the desired LFM waveform is computed by a Differential Data Analyzer (DDA), a common digital computer element. The DDA drives the phaser through the desired progression. A balanced mixer compares the phase of the VCO to that phase which is digitally generated. The difference is sampled, forming an error signal for the wideband phase-locked loop. The VCO acts as a smoothing filter and produces a desired waveform. The computation in the DDA is performed, and the phaser control is updated, at a rate that is typically 30 MHz. Since it is desired to generate LFM waveforms which are typically of 900 MHz bandwidth, the sample aperture must be less than 200 picoseconds. A sampling microwave phase detector of such small sampling aperture is considered a high-cost, high-risk component. Moreover, the phaser or digitally controlled phase shifter has a limited resolution and accuracy at microwave frequencies, and thus sets a bound on the ultimate performance obtainable with this approach. Since the phase shifting is normally quadratic, this second approach is called direct quadratic phase shifting at microwave frequency, and is described in the paper "Digital Generation of Wideband Linear FM Waveforms" appearing in the IEEE MTT-S International Microwave Symposium Digest (1980).
Before describing a third approach it may be useful to review the use of the term "LFM waveform", especially as used in the approach described below and in the present invention. "Wideband" LFM waveform refers to a microwave frequency (1000's of MHz) waveform in the form of a linearly frequency modulated carrier signal with a bandwidth of 10's or 100's or MHz. "Baseband" LFM waveform refers to a much lower frequency (10's of MHz), much lower bandwidth (MHz), waveform. Baseband LFM waveforms are employed after frequency multiplication and bandwidth expansion in order to perform linear frequency modulation of the microwave frequency carrier signal, producing the "wideband" LFM waveform. Thus, in LFM both the modulated and the modulating signals have confusingly similar names, based in the "LFM" abbreviation which is applied to both.
A third approach to the generation of wideband LFM waveforms involves direct digital synthesis at baseband, followed by modulation onto a carrier, and then followed by frequency multiplication. This approach is described in the paper "Digital Generation of Wideband LFM Waveforms" set forth in the Proceedings of the IEEE International Radar Conference page 170, et seq., (1975). As with the present invention the utility of digital generation of a LFM waveform is recognized in this approach. The generation of a LFM waveform is, both within the prior art approach and within the present invention, necessarily at a baseband, i.e. at a frequency much less than the microwave frequency at which the radar signal is transmitted. A digitally generated LFM waveform cannot be directly generated at microwave frequencies because of performance limitations in digital components. This third prior approach generates the LFM waveform at baseband, and centered on DC. The LFM waveform is then modulated onto a carrier. Only after such modulation is the composite modulated waveform converted to a microwave frequency by means of direct frequency multiplication. This approach requires in-phase and quadrature (I and Q) channels in the baseband portion of the waveform generator. Imbalances between the I and Q channels are a source of errors, and the two channels double the amount of hardware required in the portion of the waveform generator wherein such signals are developed. The modulation required is single sideband, which produces undesired image frequencies and carrier signal leakage which must both be filtered out.
Each of the above described prior art approaches has error sources that will result in the ultimately transmitted LFM microwave frequency waveform exhibiting some phase and/or frequency errors in its modulation. Low order (quadratic) frequency errors degrade range resolution by broadening the main lobe of the range-compressed radar return over a number of range cells. Higher order (multi-ripple) non-linearities in the LFM waveform result in undesirable range sidelobes on strong radar returns.
Generally these error sources cause error to accrue in all regions of the LFM signal generation, particularly in the microwave region. Because the first, linearized swept-VCO, approach, and the second, direct quadratic phase shifting at microwave frequency, approach both "compute" the linear frequency modulation in the same frequency spectrum wherein the microwave carrier signal is modulated, it is obvious that errors in "computation" of the appropriate modulation accrue to the transmitted signal. The third prior art approach introduces error in the generation (preferably transpiring by direct digital synthesis) of the LFM signal at baseband. Such errors inherent in the third prior art approach continue to accrue in the direct frequency multiplication, and in the imbalances between the I and Q channels at baseband. Those error sources will not be present within the approach to generation of a wideband LFM waveform in accordance with the present invention.
Futhermore, and importantly, there exists a problem related to the several error sources present within each of the prior art approaches to the generation of LFM signals. This problem is that the radar transmitter will itself induce phase errors. The prior art approaches, and most particularly the third approach, do not permit the recognition or the removal of phase errors introduced in the final transmitter stage. The approach in accordance with the present invention will permit removal of these transmitter-induced phase errors.